An Acyl-NHC Osmium Cooperative System: Coordination of Small Molecules and Heterolytic B–H and O–H Bond Activation

Abstract

The hexahydride complex OsH 6 (P i Pr 3 ) 2 (1) activates the C−OMe bond of 1-(2-methoxy-2-oxoethyl)-3-methylimidazolium chloride (2), in addition to promoting the direct metalation of the imidazolium group, to afford a fivecoordinate OsCl(acyl-NHC)(P i Pr 3 ) 2 (3) compound. The latter coordinates carbon monoxide, oxygen, and molecular hydrogen to give the corresponding carbonyl (4), dioxygen (5), and dihydrogen (6) derivatives. Complex 3 also promotes the heterolytic bond activation of pinacolborane (HBpin), using the acyl oxygen atom as a pendant Lewis base. The hydride ligand and the Bpin substituent of the Fischer-type carbene of the resulting complex 7 activate the O− H bond of alcohols and water. As a consequence, complex 3 is a metal ligand cooperating catalyst for the generation of molecular hydrogen, by means of both the alcoholysis and hydrolysis of pinacolborane, via the intermediates 7 and 6. ■ INTRODUCTIONCatalysts based on cooperative ligands are a promising alternative for reactions associated with conversion and storage of regenerative energy. 1 These ligands cooperate with the metal center by participating directly in a σ-bond activation stage and, subsequently, by performing the reversible structural change in the process of product formation. 2 The outer-sphere catalysis takes place with no formal change in the metal oxidation state. The heterolytic σ-bond activation is promoted by a coordinated (Noyori−Morris type) 3 or free (Shvo type) 4 Lewis base at the cooperating ligand or by means of the aromatization of the central ring of a pincer ligand (Milstein type). 5 The ability of the N-heterocyclic carbene (NHC) ligands to modify the electron density of the metallic core 6 has made the NHC transition-metal complexes powerful tools in homogeneous catalysis. These ligands are easily accessible from imidazoles, which can be functionalized on both nitrogen atoms. As a consequence, a variety of excellent promoters for enantioselective processes have been developed. 7 Because the vast majority of the NHC ligands are substituted at nitrogen by alkyl and/or aryl groups, examples of nonchiral catalysts based on NHC ligands containing an additional Lewis base are rare. 8 The scarce catalysts known of this type are centered on hemilabile systems and have been used in functionalization reactions. 9 In addition, some examples of complexes bearing protic NHC ligands acting as bifunctional systems have been also reported. 10 An added problem for the use of NHC ligands in metal−ligand cooperating catalysis is to keep the cooperation capacity between the Lewis base tethered to the imidazole moiety and the metal center, after the carbene coordination. In this context, access to NHC transition-metal complexes able to promote the heterolytic activation of σ bonds is challenging.We now show a synthetic strategy that allows keeping the cooperative capacity between the additional Lewis base, tethered to the imidazole moiety, and the metal center after the carbene coordination. ■ RESULTS AND DISCUSSIONFormation of a ...

Metal‐Free Reduction of CO2 to Methoxyborane under Ambient Conditions through Borondiformate Formation

“…The compound 6 has been thoroughly characterized (by X‐ray, NMR spectroscopy, and elemental analysis). The formation of 7 a was confirmed by NMR spectroscopy ( 1 H, 13 C and 11 B NMR data and were well matched with the earlier reported values) in case of pinacolborane (HBpin) by performing a stoichiometric experiment between 3 a and 3 equivalents of HBpin under CO 2 atmosphere, and it showed exclusive formation of 7 a′ and 6′ (see Scheme S1). The source of the oxygen atom in 7 a (Step 4) is CO 2 and the formation of 7 a can be rationalized by in situ CO elimination from the CO 2 inserted boron dimer (see Figures S2, S4, and TS5) –.…”

Tuning the Nature and Formation of Bis(dihydrogen)–Osmium Species

“…The Os-NHC and Os-acyl distances of 2.086(2) (Os-C(1)) and 2.115(2) (Os-C(6)) Å, respectively, compare well with those previously reported for other Os{κ 2 -C,C-[C(O)CH 2 ImMe]}-complexes. 14 The classical trihydride nature of the complex was confirmed by the DFT-optimized structure (Figure 4b), which affords separations between the hydrides of 1.734 and 1.778 Å. The 31 P{ 1 H}, 13 C{ 1 H} and 1 H NMR spectra in toluene-d 8 are consistent with the structure shown in Figure 4a.…”

“…13 An example is the acyl-NHC ligand of complex OsCl{κ 2 -C,C-[C(O)CH 2 ImMe)]}(P i Pr 3 ) 2 (Im = imidazolylidene), which catalyzes the generation of H 2 , by means of both the alcoholysis and hydrolysis of pinacolborane (Scheme 2). 14…”

“…In order to address this problem, we decided to prepare the methoxycarbene counterpart (Scheme 13). Treatment of dichloromethane solutions of 10 with 1.2 equiv of MeOTf, at room temperature, for 30 min leads to the expected trihydride [OsH 3 {κ 2 -C,C-[C(OMe)CH 2 ImMe]}(P i Pr 3 ) 2 ]OTf (14), as a result of the methylation of the acyl group. Complex 14 was isolated as a white solid in 58% yield and characterized by X-ray diffraction analysis.…”

“…2, 3, 7, 10, 13 and 14. X-ray data were collected for the complexes on a Bruker Smart APEX DUO (2, 3, 7 and 10) or Bruker Smart APEX CCD (13,14) diffractometers equipped with a normal or fine focus respectively, and 2.4 kW sealed tube source (Mo radiation, λ = 0.71073 Å). Data were collected over the complete sphere covering 0.3 o in ω at 100±2 K in order to minimize thermal vibration.…”